When Did I Conceive if My Due Date Is March 13?
Wondering when your March 13 baby was conceived? Using the standard 266-day rule, the answer is most likely June 20 of the previous year, give or take a few days.
Key dates for a March 13 due date
| Most likely conception date | June 20 |
|---|---|
| Likely conception window | June 17 – June 23 |
| Days when sex could have led to pregnancy | June 15 – June 21 |
| Estimated first day of last period | June 6 |
| Positive home test likely from | July 4 (around the missed period) |
| Second trimester begins (14 weeks) | September 12 |
| Third trimester begins (28 weeks) | December 19 |
| Full term begins (39 weeks) | March 6 |
How the conception date is calculated
Due dates are normally counted as 280 days (40 weeks) from the first day of the last menstrual period. In a typical 28-day cycle, ovulation and conception happen about two weeks after that period starts, so conception sits roughly 266 days before the due date. Subtracting 266 days from March 13 gives June 20.
Conception happens when sperm meets the egg, usually within a day of ovulation. Since sperm may wait up to five days for an egg to be released, the window for sex that could have caused the pregnancy is wider than a single day.
How certain is this date?
Remember that a due date is only the midpoint of a range. Ovulation timing varies from cycle to cycle, so the true conception day can sit a few days away from the estimate.
If your cycles are usually longer than 28 days, conception probably happened a little later than June 20; with shorter cycles, a little earlier. A cycle that is 32 days long, for example, shifts the likely date about four days later.
Check your own dates
Enter your exact due date in the conception date calculator to see these results with weekdays and the correct year, or start from your last period with the last-period conception calculator.
Conception Date Calculator: Find the most likely conception date from your due date.
Open the calculatorThis article is general information, not medical advice. Talk to a doctor or midwife about your own pregnancy or fertility.